trivial: Improve the ways to get bonding devices and their slaves

Compared with using glob, it could make the code simple and clean.
And it could be a little faster in the case of finding bonding device.

Change-Id: I87d25636685f75b8c429860a041494698febead0
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/13506
Reviewed-by: Dan Kenigsberg <danken@redhat.com>